The biggest promise in Plus is a granular sharing experience, the ability to craft virtual relationships that mirror the nuance of real-life ties. One of the most significant problems with early implementations of Facebook was the homogenization of degrees of acquaintance (e.g. friends vs. closest friends vs. family, etc.) into the binary categorization of "friend." Clearly, this ersatz virtual construct didn't offer the necessary mirroring of real-space relationships, and caused worlds to collide, a la the social networking cautionary tale of the day. Facebook moved toward correcting this with the creation of lists, but Plus takes it a step further.
In Plus, you add people to "circles," arbitrary collections that you can use to replicate the nuance of real-world relationships. For example, you could create a group called "high school" and put all of your high school friends in it, a group for your coworkers, etc. They also state that the group names are private, so friends are unaware of the name of the group you place them in (though I still wouldn't recommend making a group called "enemies"!). Every time you share anything (a status, a picture, etc.), you make a conscious decision as to which circle(s) you share it with. In the experience I've had so far, this is much more controlled and conscious a process than Facebook sharing through lists.
Another feature I like is that Plus stays true to Google's ethos of "data liberation." That is to say, they make it very easy to get any data you put into Plus out. You can download, for example, any pictures that you upload back to your local machine. This, I think, is actually a big deal.
In terms of design aesthetic, Plus is very austere, in the minimalist tradition of Facebook (in what I think is reactionary to the carnival chic that was MySpace). Though, to me, the feel is bordering on insipid. From an interface perspective, it seems fairly intuitive and easy to use.
While the service does offer some interesting features (video chat, group IM discussions from mobile devices, etc.), I haven't seen anything that feels revolutionary. Unless Plus is able to capitalize on a youth-led exodus from Facebook (as a reaction to the mainstream/adult presence there), I doubt that it will, at this point, make much of a dent in Facebook's market share. But I do feel like it has a better chance of being successful than Buzz or Wave (though I do think Wave was misunderstood and a good product).
Would love to hear your opinions on this.